This invention relates to a curable composition and, in particular, to a heat and/or radiation-curable protective hardcoat composition for application to an article such as a high capacity optical information storage medium.
A new form of optical information storage medium, the so-called “Blu-ray” Disc (BD) technology, has only recently made its commercial appearance. At present, a Blu-ray optical information storage disc consists of a 1.1 mm substrate layer that is sputtered on one side with a metal or metal alloy as a reflective layer, a thin information layer (for BD-ROM), a recordable layer (for BD-R) or a re-recordable layer (for BD-RE) and, finally, a 100 micron protective topcoat, or cover, layer. The cover layer consists of a relatively expensive solvent-casted polycarbonate (PC) film of approximately 100 microns thickness bonded via an adhesive to the information layer, recordable layer or re-recordable layer, as the case may be, of the substrate. Because this PC film readily scratches and acquires fingerprints, the current commercial version of the Blu-ray Disc is enclosed within a protective cartridge, a component that adds significantly to the cost of the product. The information, recordable or re-recordable layer of a Blu-ray disc is only about 100 microns below its surface therefore thus requiring increased surface integrity compared to that which is acceptable for a conventional compact disc (CD) or digital versatile disc (DVD) surface.
Efforts are currently being made to replace the protective cartridge of a Blu-ray Disc with a protective coating on the disc and even to replacing the PC film used as the cover layer with a lower cost but still effective substitute. PC film is not only an expensive material, it is difficult to assemble in the disc manufacturing process. One approach being considered to improve the Blu-ray Disc technology consists of a 2-layer spincoatable system where a first 94-98 micron layer is spun onto the information-containing 1.1 mm substrate followed by a second 2-6 micron layer hardcoat which provides abrasion resistance and anti-fingerprint properties.
Given the inherent complexities of a 2-layer spincoatable system, it would be highly desirable to combine the two coating operations into a single coating step employing a single coating composition that effectively combines all of the functions of the aforementioned two-coat system.
Abrasion resistance and scratch resistance can in general be achieved with highly crosslinked resins. However, most organic resins shrink upon polymerization. Shrinkage of the cover layer upon curing creates stress between it and the substrate to which it is applied. This stress in turn can create what is referred to as disc tilt. Because of the miniaturization of the information pits and the necessary precision requirement of the laser light, particularly in the case of Blu-ray media, excessive disc tilt must be avoided.